As for an electronically controlled automatic transmission of a vehicle, the selection of a plurality of gear position is operated by selectively actuating solenoids in response to the vehicle speed and the throttle opening. Such an electrically controlled automatic transmission is, however, impractical unless the vehicle speed and the throttle opening are accurately detected. For accurate detection of the vehicle speed, various types of abnormality determination devices for a vehicle speed sensor have been developed so far. For example, a diagnosis device for an electrically controlled transmission of a vehicle was proposed in Japan Published Unexamined Patent Application No. Sho 59-145945. In this device, two vehicle speed sensors are employed, and the vehicle speeds detected by these sensors are compared with each other. If the rate of the vehicle speeds of one sensor to the other is greater or less than a predetermined range, the device determines that at least one of the sensors has gone wrong or is working abnormally. This condition is stored in the form of a certain digital code, and then outputted to execute the self diagnosis.
One of the above-mentioned vehicle speed sensors is provided to an output shaft of an automatic transmission, and the other is installed in a speedometer unit. The latter one essentially consists of a magnet which is mechanically connected to the output shaft to be rotated together, and a reed switch for detecting the magnet. On the other hand, as shown in FIG. 7, the vehicle speed sensor provided to the output shaft is mainly made up of a rotor 304 including a gear 302 formed at the output shaft 300 and a pickup unit 310 which is mounted in proximity and opposite to the rotor 304 and includes a core 312, coils 314 and a magnet 316.
Both of the above-mentioned sensors detect the revolution speed of the output shaft. However, if racing (i.e., boosting up of the engine revolution without load is carried out when the shift position of the automatic transmission gear is set in the neutral position (N position) or the parking position (P position) and the vehicle is stationary, a slight vibration in the directio of arrows A and B shown in FIG. 7 due to the engine racing occurs to the rotor 304. The pickup unit 310 detects this vibration and generates a speed signal. On the other hand, the vehicle speed sensor installed in the speedometer does not generate a speed signal because the output shaft 300 is not actually rotating. Such an inconsistency sometimes brings about a problem that at least one of the sensors is determined to be abnormal by mistake due to the difference of the output signal, and the result of this determination is stored in a memory. Because of this, the wrong information is given when the self diagnosis is performed for the maintenance of the vehicle.